Rockwell Automation 1775-KA PLC-3 Communication Adapter Module User Manual - Manual (Page 60)

Addressing Rules and Examples
Chapter 4
4Ć4
You can also use symbols to represent data and data addresses in message
procedures. A symbol can consist of numeric digits, alphabetic
characters, and the underline character (_). No other special characters
are allowed. The first character in a symbol must be a letter of the
alphabet.
Both upper–case and lower–case letters are acceptable in a symbol, but
they are distinguished. For example, ASYMBOL and Asymbol are two
different symbols.
A symbol can be any length, but it must be unique in its first 8 characters.
For example, SYMBOL_A and SYMBOL_B are distinguishable in a
message procedure, but NEW_SYMBOL_A AND NEW_SYMBOL_B
are not. Note that indistinguishable symbols are not flagged as
programming errors. Rather, indistinguishable symbols are treated as
equivalents.
Certain words and character combinations cannot be used as symbols
because they are reserved for special uses in message procedures. The
reserved words are:
CREATE IF
DELETE ON_ERROR
ERROR PROT
EXIT STOP
GOTO UNPRO
Any abbreviated form of one of the above words is also invalid as a
symbol. For example, the single letter C should not be used as a symbol
because it is an abbreviation of the word CREATE. Similarly, PRO is an
invalid symbol.
Figure 4.4 illustrates the classification of different types of symbols. The
two major classifications are:
system symbols
user symbols
Symbols